1. Field of the Invention
The present invention relates to an image forming apparatus, a dot pattern calibration method, and a program and, for example, to an image forming apparatus having a duplicate deterrence function, a dot pattern calibration method, and a program.
2. Description of the Related Art
It is possible to guard digitized confidential information by deciding a supervisory policy for access to it and carrying out the policy. However, once the confidential information is printed on paper, restricting duplication or removal of the printed confidential information is more difficult than restricting access to the digitized confidential information. This makes security protection more difficult.
To retain the security of printed confidential information, image processing methods are becoming popular which embed restriction information to, for example, permit or prohibit copying under a specific condition, in print target image information. Restriction information is embedded in image information by encoding processing that allows decoding of the restriction information by some type of decoding processing. This processing is also used to prevent image alteration. Hence, the restriction information to be embedded is called code information or encoded information. Examples of code information are a digital watermark and an LVBC to be described later in the embodiments. These are two-dimensional code information two-dimensionally embedded in image information. The LVBC will be described later in a section <LVBC Embedding Method>.
No user can directly recognize the contents of printed code information. A user can recognize the code information only when a reading apparatus reads image data from the printed image, and the code information is decoded by decoding processing corresponding to encoding processing of the information. In general, the read image data is sent to a computer and decoded using an application program that operates in the computer. Instead, a cellular phone that supports two-dimensional codes can also read code information and decode it in the device. Alternatively, a copying machine having a plurality of functions can internally decode code information.
Once the image containing encoded code information is printed, the code information is hardly lost even if the image is repeatedly duplicated. The code information can be decoded even from the duplicated image. This enables control to, for example, prohibit a copying machine from copying an image based on decoded code information. Additionally, when, for example, information to identify a user who has printed or duplicated an image is used as code information in addition to restriction information, the user who has brought out the printed product can be identified. In this way, code information embedded in an image serves as a deterrent against duplication and alteration of printed products or bring-out of printed products by unauthorized persons.
It is understood that delicate dot thickening or thinning in a print device that prints an image including embedded code information greatly affects the code information recognition accuracy, that is, the reproducibility of decoded code information. To improve the code information reproducibility, it is necessary to use an appropriate dot size when printing an image of code information.
On the other hand, there is a technique of combining an image called a copy-forgery-inhibited pattern with a print target image in order to prohibit or deter copy in an environment where a device incapable of decoding code information from an image including the embedded code information is used. The copy-forgery-inhibited pattern is sometimes called a copy restraint pattern. A copy-forgery-inhibited pattern image is recognized by a human eye as a mere pattern or background image in an original (a printed product output by a printer based on image data) but appears as a character or image in an image obtained by copying the original by a copying machine. The original can provide the same restraint effect as anti-forgery paper. A copy-forgery-inhibited pattern image includes a portion, that is, a latent image that appears as an image upon duplication and another portion, that is, a background corresponding to the background of the latent image. The latent image and the background have almost the same average density and are hard to visually identify. The background is formed from small dots that are unreadable or are removed as noise in a duplicate. The latent image is formed from dots having a size to enable clear duplication by a copying machine. It is necessary to appropriately decide the density of the copy-forgery-inhibited pattern image, and a technique for it has also been proposed (e.g., Japanese Patent Laid-Open No. 2007-043656).
A technique has recently received a great deal of attention, which protects security in both a device which can decode code information and a device which cannot by using code information as a copy-forgery-inhibited pattern image. In this technique, a dot pattern containing code information is embedded in an image as a copy-forgery-inhibited pattern. When printing is performed using this technique, it is possible to make the dot pattern appear as a copy-forgery-inhibited pattern upon copying the original, and also recognize the dot pattern and decode the code information from it. Hence, a device capable of decoding code information executes control based on the code information decoding result. On the other hand, a device incapable of decoding code information can make a predetermined character or image appear on a duplicate.
In Japanese Patent Laid-Open No. 2007-043656, a base portion is called a background portion. In this specification, a copy-forgery-inhibited pattern image including code information is called a background image, as will be described later. Hence, the background portion of a copy-forgery-inhibited pattern is called a base or a base portion.
In the above-described code information embedding with a copy-forgery-inhibited pattern function, it is essential to simultaneously obtain an effect of a copy-forgery-inhibited pattern image and a high recognition rate of a dot pattern formed as code information. More specifically, a copy-forgery-inhibited pattern is required to include large dots and small dots, in which the small dots disappear in a duplicate, and the latent image area and the background area have almost the same average density in the original. That is, the average density range is limited to a range (also referred to as a density range) that satisfies those conditions. On the other hand, code information is required to include dots that achieve a high dot pattern recognition rate and a low visibility.
For these reasons, when priority is given to the code information recognition rate, the dot size needs to be large, and the average density of the dot pattern does not always fall within the density range required for the copy-forgery-inhibited pattern. When priority is given to the density range of the copy-forgery-inhibited pattern, the dot size of the dot pattern containing the embedded code information needs to be small, resulting in a low code information recognition rate.